The long-term goal of this research program is to provide a comprehensive understanding of the signaling of the Type I Parathyroid Hormone (PTH)/PTH-related peptide (PTHrP) receptor (PTH1R) and how it is regulated by cytoplasmic adaptor proteins such as the sodium/hydrogen exchanger regulatory factor 1 (NHERF1), also known as the ezrin binding protein of 50 kDa (EBP50). The biological action of PTH largely occurs in kidney and bone. The magnitude and duration of PTH action reflects the net balance between signal generation and signal termination by the PTH1R. Signal termination, reflected by PTH1R internalization, is conspicuously different in proximal and distal tubule cells of the kidney. For example, the agonist, PTH (1-34), internalizes the PTH1R in renal proximal and distal tubule cells. Notably PTH (7-34), which binds to, but does not activate the PTH1 R, internalizes the PTH1R only in distal tubule cells. Expression of NHERF1 in proximal and not distal tubule cells underlies the regulation of PTH (7-34)-induced PTH1R internalization. Introduction of NHERF1 into distal tubule cells blocks PTH (7-34)-induced PTHIR internalization. Bone cells also exhibit NHERF1-regulated PTH1R internalization. The parathyroid hormone related peptide also internalizes the PTH1R but there are important regulatory differences with PTH. The mechanistic differences for PTH1R internalization between PTH and PTHrP are unknown. In addition, the proteins that direct PTH1R internalization in the presence and absence of NHERF1 are yet to be characterized. To address these deficiencies, two Specific Aims have been developed. The first Specific Aim is to characterize PTH/PTHrP structure-function and the role of NHERF1 in PTH1R internalization. Specific Aim 2 is Identification of PTH1R and NHERF1 binding partners and characterization of their respective roles in PTH1R signal transduction and trafficking. These studies will provide information that will contribute to the design of therapeutic agents where cell or tissue-specific downregulation of PTH activity is desired.